As compared with widely used amorphous silicon (a-Si), amorphous (non-crystalline) oxide semiconductors have high carrier mobilities, wide optical band gaps, and film formability at low temperatures, and therefore, have highly been expected to be applied for; next generation displays which are required to have large sizes, high resolution, and high-speed drives; resin substrates having low heat resistance; and others. The carrier mobility is also called as “field-effect mobility” which may hereinafter be referred to simply as “mobility”.
Among the oxide semiconductors, an amorphous oxide semiconductor consisting of indium, gallium, zinc, and oxygen, and an amorphous oxide semiconductor consisting of indium, zinc, tin, and oxygen have been used because of their high carrier mobilities. The amorphous oxide semiconductor consisting of indium, gallium, zinc, and oxygen is occasionally referred to as “In—Ga—Zn—O” or “IGZO”
There are two types in thin film transistors of bottom-gate structure comprising an oxide semiconductor; one is an etch stop type with an etch stopper layer 9 as shown in FIG. 1A, while the other is a back channel etch type without an etch stopper layer as shown in FIG. 1B. Hereinbelow, the etch stop type and the back channel etch type are referred to as “ESL (Etch Stop Layer) type” and “BCE (Back Channel Etch) type”, respectively.
The BCE type TFT, without an etch stopper layer, depicted in FIG. 1B is superior in terms of productivity because formation of an etch stopper layer is not necessary in its fabrication process. The BCE type TFT may have a structure depicted in FIG. 2 in which a passivation layer 6 in FIG. 1B is a double layer.
There is a problem, however, in the fabrication process of the BCE type TFT as described in the following. A wet etchant for example an acid-based etching solution including phosphoric acid, nitric acid, and acetic acid, is used for patternining a source-drain electrode formed on top of the oxide semiconductor layer. A surface of the oxide semiconductor layer being subjected to the wet etchant is etched or damaged so that the TFT characteristics of the oxide semiconductor may be deteriorated.
An oxide semiconductor consisting of aforementioned IGZO, for example, shows an high solubility to inorganic acid-based wet etchants which are used to wet etch source-drain electrodes, and is extremely easily etched by the inorganic acid-based wet etchant solutions. If the IGZO film is dissolved in the wet etching process of the source-drain electrode, fabrication of TFT then becomes difficult, and the TFT characteristics are deteriorated. Hereinbelow, etching by using an acid-based wet etchants is occasionally referred to as “acid etching” or “wet etching”.
In an attempt to suppress the damage to the oxide semiconductor layer of the BCE-type TFT, for example, technologies of Patent Documents 1 to 3 listed below have been proposed. These prior arts propose to suppress the damage to the oxide semiconductor layer by forming a sacrificial layer or a recessed part between an oxide semiconductor layer and a source-drain electrode. It is necessary, however, to increase numbers of processing steps in order to form such a sacrificial layer or a recessed part. Further, the non-patent Literature Document 1 discloses removing a damaged layer from the surface of the oxide semiconductor layer. It is difficult, however, to uniformly remove such a damaged layer.